A taxonomic revision of the genus Casselia is provided. Casselia is a small genus of Verbenaceae, placed in the tribe Casselieae, along with Tamonea and Parodianthus. It is represented by six species, distributed in South America, along central Brazil, eastern Bolivia and northern Paraguay. Five new synonyms and two lectotypifications are proposed. Detailed morphological descriptions are given for each taxon, as well as a key for their identification, illustrations, actualised synonymy, distribution, and a discussion about the relationship between closely related taxa.